For improvement of detection accuracy of a pressure-sensitive sensor, the following is known as a technique for reducing variation in pressure-sensitive sensor characteristics between individuals.
Namely, there are known a technique to determine an approximation expression representing a relationship between output and pressure for each pressure-sensitive sensor on the basis of an actual measured data (for example, refer to Patent Document 1) and a technique to determine standardized information of external force-resistance characteristics in which a resistance value of a pressure-sensitive sensor is considered to be 0 when an external force is 0 and the resistance value of the pressure-sensitive sensor to be 1 when an external force is at its maximum (for example, refer to Patent Document 2).